Youkilis wasn't stuck in AAA in any meaningful sense.
In 04, Youk managed only a .753 OPS in AAA, and they still called him up for backup duty and he wound up with about 250 PA's that year. Managed some pretty solid numbers but nothing really stood out, so they brought in some veterans who beat Youk in camp. La de da, life in a franchise that proceeds cautiously with young players when given a choice.
In 05, he started the season in AAA, lit it up, and added himself to the mix in Boston despite Mueller, Millar, and Olerud dominating the depth chart ahead of him.
After that, he was full time in the bigs.
The closest thing he ever was to seriously "stuck" was the first half of 05, and that's a decision you make when a rookie is good-not-great and you're trying to win a second World Series. And even then, he was allowed to make a case for himself when every other option was no better than he'd proven he could be, and won himself playing time as a result. All he had to do in the end is make the best use of the chances he was given to make himself an option in the eyes of the franchise. With that accomplished, a path to the bigs would eventually open for him. And it did.
Try again.